There are many applications in joinery in which two workpieces must be secured together in such a way that the fastening elements are not visible from the outside, and sometimes the inside, of the resulting structure. One such application is in connection with ready-to-assemble or knock-down furniture and casework. Such furniture is manufactured using high-volume, wood-working production equipment and is sold to the ultimate user in a ready-to-assemble condition. A relatively unskilled purchaser must be able to assemble the components using simple and inexpensive tools into a sturdy piece of furniture.
To facilitate the assembly and alignment of parts, ready-to-assemble components sometimes include alignment dowels and mating bores. While easing the purchaser's assembly task, dowel-based ready-to-assemble furniture inherently is more expensive than ready-to-assemble furniture and casework in which the joint between components effects alignment of the components.
Ready-to-assemble furniture is often formed from particleboard or hardboard which has a wood or wood-simulating veneer or finish on one or more surfaces. Particleboard, while being a relatively inexpensive and yet strong material, has several characteristics which makes its use in ready-to-assemble furniture difficult. First, particleboard has very hard opposed surfaces, but the mechanical strength of particleboard drops rapidly in the center or away from the opposed surfaces. Second, the appearance of particleboard when cut through the surface veneer or finish is very poor.
Various types of wood-working joints have been employed in the construction of ready-to-assemble furniture. In some systems one or two fittings or inserts are mounted in the workpieces and fasteners, cams or latches used to join the fittings, and thus workpieces, together. In other systems a screw-type fastener extends from a cavity or mortise in one workpiece and is screwed into the other workpiece. Typical of the fitting-based systems are the ready-to-assemble furniture joints in which a fastener is secured to one workpiece and a fitting or insert in the other workpiece cooperates with the head of the fastener to join the two components together. Such fastening systems are shown, for example, in U.S. Pat. Nos. 4,728,215, 4,669,910, 4,579,474, 4,576,506, 4,564,306, 4,559,011, 4,553,873, 4,545,698, 4,505,610, 4,487,522, 4,408,923, 4,360,282, 4,353,663, 4,341,485 and 4,202,645. A cam or second screw is used to engage the head of the fastener and produce camming against the head of the first fastener to pull the fitting up snug against the first fastener head.
In addition to the cost and complexity of such fittings or fastening inserts, forming the joint requires at least two fastening steps, namely, orienting the camming member in the fitting to apply a force to the first fastener and applying the fastening force against the head of the first fastener. Moreover, cam-based joints inherently have the problem of having a trade-off between size and the throw or distance over which the cinching-down occurs. As the size of the cam assembly, and scar on the workpiece, decreases, the throw of the cam assembly decreases. Thus, large cam assemblies can have relatively large throws, but the small ones do not and accordingly more precision in the location of the parts must be employed. By contrast, a screw-type fastener generally can be very small in diameter with a long fastening throw.
A further serious problem which occurs with systems in which a fitting is secured to a fastener head is that the complexity of the fitting requires that a relatively large cavity or mortise be formed immediately adjacent to or abutting an edge of the workpiece, as is shown, for example, in U.S. Pat. No. 4,353,663. This cavity greatly weakens the workpiece at the fitting, particularly if particleboard is employed. The relatively low strength of particleboard in the interior volume of the board can result in breaking away of the edge weakened by the fitting receiving cavity.
Similar problems are encountered in joints of the type shown in U.S. Pat. No. 4,357,119 wherein large fittings, which snap into coupled relation, are mounted close to the edges of the workpieces.
More complex ready-to-assemble furniture joints are shown in U.S. Pat. Nos. 4,664,548 and 4,361,931 in which latch mechanisms are employed. These latches similarly have a relatively short throw during fastening, require large fittings at the edges of the workpieces and are inherently undesirably expensive.
Another approach in ready-to-assemble, fitting-based joinery is to employ two fittings with one mounted in each workpiece. The fittings interengage each other or are jointed together by a cam-acting locking member. Typical of such fittings are the assemblies of U.S. Pat. Nos. 4,348,130, 4,325,649, 4,272,207, 4,160,610 and 3,836,267. These joints, however, again are undesirably complex and require large cavities next to the edges of the components being joined.
In some joints the fastening fittings or inserts are external, that is the coupling together of the fittings is accomplished outside the volume of the workpieces being joined. Thus, a relatively small volume is required to joint each fitting to its workpiece, but the cam or snap-acting coupling structure protrudes from inside or outside of the structure, which can be unsightly and obstructive. U.S. Pat. Nos. 4,634,309 and 4,292,003 show such external joints.
Another approach which has occasionally been employed is to position the fastening fittings at a distance from the edges of the workpieces and employ long pins or bars which extend between the workpieces. U.S. Pat. Nos. 4,405,253 and 1,602,658 illustrate such furniture joints. They require, however, long bores in the workpieces and the manipulation of several pieces to effect coupling together of the furniture components.
Since cam-based fittings have various disadvantages, as above set forth, efforts also have been directed to the use of joints in which screw fasteners can be used as the basic fastening mechanism. Screw fastener-based joints have the advantage that they can employ the relatively long throw of a screw during fastening to reduce the manufacturing precision required for the joint.
In U.S. Pat. No. 4,639,161 two fittings with two screws are required to make the joint. Since the screws are mutually perpendicular, the advantage of a longer throw is minimized. Moreover, a large insert cavity or notch is required at the edge of one of the workpieces.
In U.S. Pat. Nos. 4,236,848 and 4,089,614 single screws are employed, but the fittings again must be mounted in a large recess which abuts the edge of one of the furniture components. Weakening and unsightly scarring of the workpiece results, and in U.S. Pat. No. 4,089,614 a screw-receiving fitting also is required.
The basic screw-type fastening joints are typified by U.S. Pat. Nos. 4,603,719, 3,675,312, 3,496,974, 1,602,658 and 1,335,544.
In U.S. Pat. No. 3,496,974, for example, a joint is formed using a stepped drill bit which produces a mortise and a screw receiving bore in the inner surface of a wooden member proximate an edge of the member. The mortise and the screw receiving bore are both formed at an angle with respect to the surface of the member being drilled, and the joint is generally referred to in the joinery industry as a "toe mortise" joint. A screw is inserted through the mortise and the screw receiving bore and extends outwardly of the edge of the member to permit driving of the screw into a second member. As shown in the '974 patent, the members are oriented in substantially the same plane and joined together in abutting relation. It also is possible to use this toe-fastening technique to join one member in a perpendicular orientation to another member.
Although such toe mortise joints are advantageous in many respects, one disadvantage of the method and apparatus for joining workpieces of U.S. Pat. No. 3,496,974 is the difficulty in providing a bore which is at a slight angle with respect to the surface of the workpiece. For particleboard and hardboard, as well as very hard woods, drilling at a shallow angle, even with jigs or fixtures, is almost impossible. The drill will tend to wander, be deflected toward a parallel orientation with the surface as it enters, and the drill bit may break as a result of such deflection. Moreover, the counterbore that is drilled is very often ragged and characterized by chipping.
U.S. Pat. No. 3,675,312 discloses an alternative joint forming apparatus and process for joining two members together in perpendicular relation to one another. This approach, however, also utilizes an obliquely directed access passageway and a fastener receiving bore that is parallel to the surfaces of the member. The passageway terminates in a large counterbore which is drilled into the edge of the member which will carry the screw. An annular insert having a bore for receiving a fastener is adhesively secured in the counterbore, and the access passageway is used to drive the screw into the second member. PG,9
The approach shown in the '312 patent has the disadvantage that the separate insert piece must be adhesively secured in the counterbore by a separate process. Moreover, the time and effort required to drill the necessary passageways and to fasten the members is undesirably long.
Other even more complex face-framing techniques are taught in U.S. Pat. Nos. 1,335,544 and 1,602,658.
In my U.S. Pat. No. 4,603,719, the teachings of which are incorporated by reference herein in their entirety, an improved apparatus and method for forming a mortise and counterbore are shown. This mortise joint is suitable for use in a variety of materials including hard woods, particleboard, hardboard, plastics, laminated plastics, and other substrate products, and can be used for ready-to-assemble or knock-down furniture joints.
Instead of using a drill at a shallow oblique angle, as shown in U.S. Pat. No. 3,496,974, my '719 patent teaches a method whereby a mortise is formed using a router. The router, rotating about an axis nearly perpendicular to the surface of the member being mortised, is mounted on a carriage which pivots or slides the router bit into contact with the workpiece to produce an inwardly tapered or slightly curved mortise or cavity in a surface proximate the edge of one workpiece. A fastener receiving counterbore is formed by a drill in a second operation.
In the preferred form of the '719 patent, the counterbore is drilled from an edge of the workpiece and extends inwardly to the mortise at a slight angle to the surface of the workpiece, thus producing a toe mortise joint. The same apparatus and method, however, also may be used to form a "pocket mortise" joint, that is, a joint in which the counterbore is parallel to the surfaces of the workpiece. A fastener, such as a screw, then may be inserted into the mortise and counterbore and fastened to another workpiece to securely join the workpieces in a parallel or perpendicular relationship.
If the counterbore in the '719 patented joint is at a slight angle, for example, seven degrees, to the surface in which the mortise is formed, a standard screw driver can be used to secure the two workpieces together. The toeing or slight angle of the counterbore, however, makes the alignment of the counterbore with a pre-drilled bore in the second workpiece relatively difficult, particularly in high-volume production runs. Accordingly, in the preferred form of the mortise joint of my '719 patent, self-boring fasteners are used, and a standard screw driver is used to drive the screw into an unbored second workpiece.
This use of self-boring screws is acceptable and advantageous for many applications. It has the disadvantage, however, of requiring a relatively long mortise to provide sufficient scope to enable a screwdriver to apply sufficient axial load to the screw to effect self-boring. Moreover, it is relatively difficult for an unskilled homeowner or operator to accurately hold the workpieces against movement as a unit and against relative separation or movement apart while driving the self-boring screw. Additionally, the fine pitch threads of self-boring screws require more time for the operator to drive them and result in more operator fatigue. Since self-boring screws are difficult to drive, there also is a tendency to torque down the screw too much and break out the edge of the workpiece. Since self-boring screws are relatively narrow or small in diameter, e.g., 3 millimeters, shock loading of the joints, for example, when the furniture or casework is bumped, can cause the small diameter screws to crush the lower strength cores of particleboard. This can result in a loosening and eventual failure of the joint.
If a parallel counterbore is used in my '719 patent to form a pocket mortise joint, a specialized tool, such as a screw driver having a flexible shaft, must be employed to apply sufficient axial force to the fastener to cause the screw to be self-boring. The homeowner, therefore, must be given the specialized flex-shaft tool with the ready-to-assemble kit, which also increases the cost of the resulting furniture.
Thus, prior joinery techniques have been found to require various compromises when applied to ready-to-assemble furniture. The present invention seeks to overcome these disadvantages and to provide an improved mortise joint for ready-to-assemble furniture and casework.
Accordingly, it is an objective of the present invention to provide an apparatus and method for securely fastening two workpieces together which is especially suitable for use in ready-to-assemble furniture or casework and which provides simple assembly and rapid fastening of the workpieces, requiring no specialized tools or skills.
It is another objective of the present invention to provide an apparatus and method for joining two ready-to-assemble furniture and casework components together with is adaptable to high-volume, low-cost manufacturing techniques.
Another objective of the present invention is to provide a ready-to-assemble furniture and casework joint having improved strength, reduced surface scarring and simpler parts.
Still another objective of the present invention is to provide an apparatus and method for securely fastening two workpieces at a joint in which the fastening stresses are more evenly distributed over the face of a mortise to substantially eliminate large, concentrated stresses.
A further object of the present invention is to provide a ready-to-assemble furniture joint which can be assembled in less time using simple and inexpensive tools.
The fastening joint and method of the present invention have other objectives and features which will become apparent from, or are set forth in detail in, the Best Mode of Carrying Out the Invention.